Pulse generator utilizing bombardment induced conductivity



K. G. M KAY Nov. 22, 1955 PULSE GENERATOR UTILIZING BOMBARDMENT INDUCED CONDUCTIVITY 2 SheetsSheet l Nuns/v Filed Dec. 50, 1950 SCOPE AMPLIFIER! FIG. 3

vr 0T r M m m M M. Am 1 l MK M 5 2 TC T a w 3 NM T MC A m o WG N m m n K W M M w 5 V W M H um x %E# 0 n 0 m u A 4 .H 6 F 50 M m 4 m a s K. G. M KAY Nov. 22, 1955 PULSE GENERATOR UTILIZING BOMBARDMENT INDUCED CONDUCTIVITY 2 Sheets-Sheet 2 Filed Dec. C50, 1950 M m w W 0 L T m M R u M w m um um m m no a l/VLZUCEO COIVDLCWV/TY AND 20 SECONDARY EMISSlV/TY HIGH ENERGY BOMBARD/NG I. BEAM SWEEP. CIRCUIT L SYA/CHRON/ZI/VG LEAD INVENTOR KGMc/(AV BY ATTORNEY United States Patent I O PULSE GENERATOR UTILIZING BOMBARDMENT INDUCED CONDUCTIVITY Kenneth G. McKay, Summit, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York This invention relates to bombardment induced conductivity in solid insulators and more particularly to electrical pulse generators utilizing such induced conductivity. This application is a continuation-in-part of my application Serial No. 789,667, filed December 4, 1947, issued February 27, 1951 as Patent No. 2,543,039.

In the parent application Serial No. 789,667, I disclose an improvement consisting largely in the use of an alternating rather than a direct voltage field across a bombarded solid insulator, together with bombardment of said insulator during all or a part of both positive and negative half cycles of the alternating voltage; or, alternatively, for achieving a like effect, the use of a very thin solid insulator in conjunction with a very high field across the same. The improvement is without regard to the particular kind of radiation concerned so as, therefore, to be applicable to the use of alpha particles, beta particles, electrons, mesons, X-rays, or gamma rays, among others.

The phenomenon of bombardment induced conductivity in solid insulators is an instance of valve action. Analogously, as a vacuum tube is made conducting under the influence of electrical means independent of the voltage applied between the electrodes, in the present studied phenomena, a normally insulating solid material is made conducting by the incidence of bombardingcharged particles undercontrol of conditions specific to the bombarding function rather than to the electric field induced by the electrodes bounding said solid insulator.

Similarly, as a charged particle of a conventional type, as alpha, beta, or electron particles, of suflicient energy can remove a valence electron from its bonds, so

also units (photons) of electromagnetic radiation, as in.

X-rays and gamma rays, may possess sutficient energy to cause the removal of valence electrons from their bonds in such a Way that the solid insulator is rendered temporarily conducting.

The bombarding particles penetrate the insulator, causing a disruptive separation of the positive and negative charges specific to the atoms which are effected by said bombarding particles. These charges are drawn toward the electrodes by the potential therebetween, which sets up an electric field in the insulator; this motion of charges constitutes a conduction current which may be suitably amplified and measured by conventional apparatus.

The material chosen for the solid insulator should have a high insulating characteristic so as to be most amenable, without ambiguity, to the conditions imposed by the type of phenomena being treated. To this end, and for other reasons as well which are not fully known at this time, the insulator should have preferably good insulating qualities and also should be preferably of a single crystal type with a high degree of chemical purity and freedom from inelastic strain or other crystal defects.

These considerations commend the use of diamond, quartz, zinc sulfide, the alkali halides (including potassium chloride and potassium bromide), magnesium oxide,

2,724,771 Patented Nov. 22, 1955 calcium fluoride, sodium nitrate, topaz, silver chloride, orthoclase, beryl, calcite, apatite, selenite, tourmaline, emeralds, extremely pure silicon carbide, and stibnite. Several of these substances, notably diamond, zinc sulphide, magnesium oxide, silicon carbide, stibnite, have been used in the basic studies of bombardment induced conductivity; and there is every good reason to think that the feature of using an alternating voltage field, attributable to applicant, is applicable to each of them. This feature has been used with eminent success with a diamond insulator, using electrons as the bombarding particles. Alpha particle bombardment of diamond, zinc sulphide, and magnesium oxide also has been used in the basic work in this art, and applicant has found that the operation is improved by the use of the alternating voltage field of his invention and has excellent reason to think that a similar improvement would be obtained under beta or meson particle bombardment and under irradiation by X-rays and gamma rays.

Diamond is'a favored solid insulator for this work because it can easily be obtained without suflicient impurities or imperfections to affect its high insulation resistance or its conducting properties under bombardment. The carbon atoms therein consist each of a nucleus exhibiting fixed units of positive charge, to which two. electrons are tightly bound. This core is surrounded by four valence electrons. The nucleus weighs twenty-two thousand times as much as an electron. The carbon atoms are held together by electron pair bonds between adjacent atoms. The insulation resistance is high because the electrons bonds are very tight. As a result of this tightness, very few electrons are displaced from their bonds by thermal agitation. This is not the case in, for example, metals, where a large number of electrons are continuously being displaced by thermal agitation and are relatively free to wander through the metal. This, under adequate conditions, constitutes the usual current in a metallic conducting medium.

When charged particle bombardment removes a valence electron from its bonds in an insulating target producing a deficiency of one electron in the atomic structure immediately affected, this localized electron deficiency is called a hole. Under an applied electric field, the arrangement of the electrons is changed, and the location of any given hole will change. As a consequence, the hole may be conveniently regarded as a positive particle which is free to move under the influence of the field. Similarly, the electron freed from the bond in question constitutes a negative particle which is free to move under the influence of the field. If there is no applied field, any free electron or positive hole moves in virtue of thermal agitation and consequently has a completely random motion. Under an applied electric field, there is a directional motion superposed upon the random one. The order of mobility of the electrons in diamond is 1000 centimeters per second for a field of one volt per centimeter. For a field of 10 volts per centimeter, the velocity, therefore, is 10 centimeters per second. For a diamond crystal one millimeter thick, the transit time, therefore, would be 10' seconds. The mobility of the electrons is aifected by the number of traps, that is, the presence of foreign atoms or imperfections, in the crystal. If an electron gets into a trap, it takes a greater or less amount of time to get out, depending upon the thermal energy required. If the time which a free-electron spends moving inthe crystal before being trapped is, on the average, less than the transit time, many of the electrons freed by the bombard-- be less than if the electrons had been collected on an electrode. Similar considerations concerning mobility and trapping also apply to conduction by positive holes. In order to minimize the number of effective traps in a given target so as to realize a substantial conductive current, the length of path in the target between the electrodes should be made as small as possible.

Inaccordance with the present invention, there is provided an improved form of pulse generator of the bombardment induced conductivity type in which an alternat ing voltage is established across the insulator target or in which, alternatively, a very high electrical field is employedin conjunction with a very thin insulator target.

One embodiment of the invention described in detail hereinafter comprises an evacuated envelope including an electron gun for producing a directed beam of bombarding radiation of any of the types described, in the path of which beam is. disposed an insulating crystal element having electrodes coated or plated on its opposite faces, to one of which is connected an alternating voltage source and to the other of which is connected an energy utilization circuit of some conventional form. Interposed in the path of the beam so as to protect the inner face of the insulator target is a shield which is impervious to the radiation of the beam with the exception of a small aperture in its central portion. The sweep of the beam back and forth across the target is controlled by a pair of de' flectingplates connected to a sweep circuit. This results in a short pulse being impressed on the utilization circuit whenever the beam sweeps across the aperture.

'In accordance with a modification of the aforesaid embodiment, it has been found advantageous to replace the bombarded electrode contact by a low impedance conduction path provided by secondary electrons produced in the insulator target by a second beam and flowing to a collector electrode which is electrically coupled to the utilization circuit. For this purpose, a low voltage high current beam is employed, which is broadly focused on the inner target surface in addition to the beam of bombarding radiation.

A satisfactory theoretical picture explanatory of the advantages derived from the use of alternating as compared with direct voltages, as in accordance with the invention, is not now completely available. It is, of course, recognized that such an explanation or a physical picture of the operation in question, is not necessary to support the present specification and claims under the patent statutes. However, it is evident that the adverse condition which tends to be, and is, remedied by the use of the alternating voltage across the solid insulator electrodes is of 'the nature of a polarization or a space charge; that is,

an accumulation of a net excess of either positive or nega .tive electrical charge in a certain region .or regions of the crystal. The following rough hypothesis, which is amply justified by observations so far, may be helpful.

Immediately after applying a steady voltage, most of the electrons which are freed by bombardment of the surface layer just beneath the thin cathode (negative electrode) move'through the crystal from'their point of origin near the cathode and are collected on the anode (the other electrode). However, some do not. Presumably, these latter are trapped and rendered temporarily .immobile by imperfections or'impurity atoms in the body of the crystal. Electrons, therefore, tend to accumulate in regions of the crystal other than the very thin layer near the bombarded surface where .they are freed. This is true,

of'course, without regard to whether the electrodes are in side-by-side presentation or in opposite presentation with respect to the intervening crystal Tbody. Thecrystal is then said to be polarized, that is, this accumulation of negative charges in the region'betwecn the source of the electrons and the anode opposes the force of the latter in attracting electrons away from this source. This elfect is cumulative so that with the passage of time, newly-freed electrons are unable to move -far from their source, and

only a small conduction current is observed. It is in this sense that the effective yield of internally freed electrons is observed to be relatively low with a steady voltage across the crystal.

This undesirable situation is disturbed if the applied voltage is reversed and the crystal is again bombarded. Now, positive holes, instead of electrons as before, move across the crystal toward the back contact of the crystal which before constituted the anode but after reversal of the voltage would tend to function as the cathode. Some of these are trapped similarly as the electrons in the earlier phase, thus setting up a positive space charge or polarization tending to neutralize the negative charge or polarization of the first phase, although some of the incomplete atoms which give rise to the positive holes may actually recombine with the trapped electrons. In either case, the negative space charge set up by the trapped electrons is greatly reduced or eliminated, and the further reversal of applied voltage to restore the initial phase will thus cause newly freed electrons to move across the crystal until the opposing space charge again begins to form. Thus, if an alternating voltage of sufiiciently high frequency be applied across the crystal in the bombardment, there is not time for an appreciable space charge to accumulate before the voltage is reversed and the space charge is partially or completely neutralized. Hence, the effective yield of electrons is relatively large at all times when the applied voltage is such as to cause them to flow across all, or an appreciable part, of the crystal, providing this voltage alternates at a sufiiciently high frequency. It has been determined that under certain experimental conditions, a frequency of 20 cycles or greater is adequate.

For optimum space charge neutralization, the extent of the primary bombardment, both in time and intensity, during the negative half cycle of the alternating voltage must be adjusted with relation to the extent of the primary bombardment during the positive half cycle. It may be desirable to use a direct potential bias superimposed on the alternating voltage, since this would render the peak voltages of the positive and negative half cycles different in absolute magnitude and that this tends to result in a more homogeneous neutralization of the space charge throughout the thickness of the crystal. The reason for such a bias, at least so far as concerns this latter effect, is, of course, based on the dilference between electrons and positive holes in their probability of being trapped.

Throughout the above discussion, it has been assumed that the penetration of the primary bombarding particles is negligible. However, in a sufficiently thin crystal, this is not true, and the current flow from the point of origin back to the bombarded crystal face becomes important. The above argument still applies to this condition, however, except that account, of course, has to be taken of the current implied by the return of electrons or positive holes from their points of origin back to the bombarded face-and the corresponding neutralization of space charge which this brings about.

The line of argument above is equally relevant to the specific applications to be disclosed in detail below. It must be emphasized that the -use of the term alternating voltage should be interpreted in its broadest sense as applying not only to a sinusoidal wave form but also to other recurrent wave forms, such as square waves or more complex forms. The-principal requirement .on the fieldapepars to be that at a certain-critical time, the field across .the crystal must be in a certain direction and that at some later critical =time, the field should be in the .opposite direction, these times being correlated with the extent of primary bombardment. The choice of types of alternating voltage wave forms becomes significant, for example, in the application of an alternating field to a crystalbombardedxby .alpha particles. :Because ofthe oidal modulation is'not particularly applicable, although square wave modulation is found to be very useful.

Another method of overcoming the effects of space charge is to use very thin crystals in conjunction with high fields applied across the same. For added effect, the field may be an alternating voltage field. It is contemplated that an extremely high field across the thin crystals might actually nullify the effects of space charge even when the field is a direct voltage field. For example, a field ranging as high as that which will cause dielectric breakdown under bombardment (of the order of volts per centimeter) might be applied between electrodes separated by from 10* to 10* centimeters. In this case, such a field would be so large that, even if all the traps in the crystal were full, the resulting opposing space charge field would be small by comparison. Moreover, if the thickness of the crystal is of the same order of magnitude as the depth of penetration of the particles of the primary beam, currents of electrons and positive holes will accordingly be traveling in opposite directions in the same region in the crystal and will thereby tend to neutralize the accumulated space charge.

With reference to the use of alternating voltage with electron bombardment induced conductivity, it is desirable to estimate the order of magnitude of alternating voltage and frequency that can be applied across the crystal and be expected to yield useful results. The figure of 20 cycles per second suggested above is in contemplation of the use of diamond as the crystal substance, although there is reason to think that comparable values would pertain to other solid insulators adaptable for this purpose. The same is true of the figures now to be presented.

The limits to be imposed on the voltage and frequency tend to be functions of the bombarding current, the induced conduction current, and the geometry of the crystal. Nevertheless, it would appear feasible when using small values of the above currents to go down to frequencies of a few cycles per second, that is, considerably less than the above indicated 20 cycles per second. The upper frequency limit will probably be determined by the electron transit time between the electrodes. Thus, frequencies of the order of 10 cycles per second are certainly practicable, and probably frequencies of as large as 10 cycles per second could be used. Usable field strength across the crystal will probably have a lower limit of the order of 10 volts per centimeter. The upper limit will probably be set by dielectric breakdown of the crystal, which would tend to occur at around 10 volts per centimeter. in terms of practicable crystal electrode separation, the actual applied voltages should range from something less than 100 volts up to several thousand volts. The useful bombarding voltage range, that is, range of energies of primary electrons, will probably run from something less than 1000 volts up to many kilovolts. Applicant and his confreres commonly used from 10 to kilovolts, although there is reason to think that it would be practicable to go to very much higher voltages.

Other objects and teachings of the invention are de rivable from the detailed description hereinaftenfollowing, with reference to the accompanying drawings, m which:

Figs. 1 and 2 illustrate two preferred methods of applying the necessary alternating difference of potential (alternating voltage) to the surfaces or parts of surfaces of the insulators in question, with relation to the incidence of the bombarding particles;

Fig. 3 illustrates a system for indicating the presence of conductivity in an insulator which is affected by the bombardment of charged particles;

Fig. 4 illustrates a pulse generator utilizing the bombardment induced conductivity principle in accordance with the invention; and

Fig.5 is an alternative form of the pulse generator circuit illustrated in Fig. 4 in which the inner connecting electrode of the bombarded insulator is replaced by beam stimulated secondary radiation from the said insulator and a collector for the same.

As has been said, the incident ray or beam which produces, by bombardment thereof, induced conductivity in a solid insulator (diamond or the like), may almost impartially be made up of any one of various common types of radiation. This includes ordinary electrons as typified by cathode emanations in the usual electronic devices, beta particles which are essentially high speed electrons, and alpha particles which are positively charged particles. Alpha and beta particles usually, and as contemplated by the present disclosure, emanate from radioactive material.

Figs. 1 and 2 illustrate two kinds of electrode systems that may be almost impartially used in any of the systems above described, although a particular choice may be urged by particular practical considerations. These two systems differ in the nature of the coupling of the electrodes to the solid dielectric substance on which they are superposed. In Fig. l, the two electrodes are mounted in a side-by-side presentation on the same surface of the solid insulator in question, which will be here assumed to be a diamond as in other figures unless specific notice is given to the contrary. In this arrangement, the conduction current flows only near the bombarded surface of the diamond, whereas in Fig. 2, the electrodes are mounted on opposed surfaces of the diamond so that the conduction current represents a phenomenon existing throughout the mass of the diamond.

Referring to Fig. 1 more specifically, two conducting metal film electrodes 1 and 2 are mounted on one surface of the insulator 3. The gap 4 separating the electrodes is relatively small and various widths from .001 to .008 inch have been sucessfully used in bombardment induced conductivity tests.

These electrodes may be prepared by dividing the diamond surface roughly in half by stretching a wire of appropriate diameter across and in close contact with the surface and then evaporating a conducting metal layer, in vacuum, onto said surface. This layer can be made so thin as to be semitransparent, provided its electrical resistance is so low as not to affect its electrical performance unfavorably. The shadow cast by the wire provides a gap when the wire is removed. This gap would have constant width and represent a uniformly high resistance thereacross at any point.

The charged particles are assumed to conform to a ray or beam indicated generally by reference numeral 5, which beam is incident on the diamond surface. Of

course, the beam tends to be most effective where it Is incident on the diamond surface at the gap but, depending on the type of charged particles, the electrodes would not necessarily impose a substantial barrier; however, the electrode system of Fig. 1 requires that the bombarding particles strike the gap or very closely adjacent thereto. Later numbered figures will show, most specifically and in detail, organizations including the elements which are here shown to a large extent diagrammatically. The angle of incidence is not critical.

Moderate alternating voltages applied between these electrodes by source 6 produce relatively high alternating electric fields in the top surface layers of the diamond, and the resultant induced conductivity pulses observed in the indicating means, which is diagrammatically indicated as a meter, pass across only these surface layers. In the statement of invention above, certain quantitative values, or their criteria, have been indicated, this applying not only to this figure but to the other figures as well.

Fig. 2 presents a second type of electrode placement. Here, the electrodes 1 and 2 are placed on opposite sides of the diamond 3. A typical diamond specimen for this purpose might be about one-quarter inch in either principal dimension and about .020 inch thick. Thus, a potential difference of volts from alternating voltage source 6 across these electrodes will produce a uniform electric field of about 2000 volts per centimeter throughout the body of the diamond. In this type of electrode placement, the induced conductivity pulses, observed in the meter indicating device shown, pass in alternate directions through the body of the diamond, as distinguished from the Fig. 1 placement in which the pulses pass in the region of the front surface and alternately in directions along said surface.

In Fig. 3, illustrating a practical embodiment of a system operating according to the principles enunciated with respect to Figs. 1 and 2, like elements are again designated by like reference characters. The diamond 3 is coated with metallic electrodes 1 and 2, as in Fig. 2. The whole is mounted in an evacuated receptacle 7. The charged particle source 8, first assumed as the source of alpha particles, may consist of a silver sheet 9 on which is deposited a layer of radium sulphate having a given density of radium atoms (in a typical instance, 12 micrograms of radium per square inch). Of course, other sources of alpha particle emanations are well known in the art and may impartially be used in the Fig. 3 organization. In fact, said organization may well be used to explore the possibilities as to new sources of said emanations. The reference numeral 10 indicates diagrammatically a support for the silver sheet. In the prior art, there are adequate teachings of mountings similar to this and the other elements here disclosed in an evacuated container. Other facilities, likewise taught by the prior art, could be used to advantage, such as a magnetic control means to determine the particular direction of incidence of the particles on the diamond or even to adjust the position of the alpha particle source in apposition to the aperture 11 in diaphragm-like element 12 for further determining and limiting the precise coaction of the beam of charged particles and the diamond.

The same illustration is applicable to the use of a beta particle source, and in this instance, the element 9 could have the form of a pliece of glass on which a minute quantity of artificially radioactive strontium has been deposited. The same teaching etxends, of course, to other sources of charged particles or electromagnetic radiation such as gamma or X-rays.

In this figure, the alternating-current source 6 functions similarly as the like numbered source in Figs. 1 and 2 to apply the desired voltage across the diamond, that is, between the electrodes thereof. To suit the teaching of this figure, which discloses a more elaborate and complete organization than that of Figs. 1 and 2, the potentiometer 13 may be used, as shown, to determine a desired fractional part of the voltage of the primary source, the voltage impressed therefrom being indicated by the voltmeter V. Of course, in the specific instance of Fig. 3, the bombarding particles penetrate the exposed electrode before affecting the diamond, this, of course, not representing a significant departure from the alternative in which the diamond is directly bombarded, providing this electrode be sufiiciently thin. The detecting circuit may comprise amplifier 14 and cathode-ray oscilloscope or the like 15, both shown diagrammatically to suggest the comparatively impartial choice of specific means to achieve these functions.

It is not a rigid requirement that the container be evacuated. In practice, a rough vacuum is produced merely to eliminate small induced conductivity pulses caused by ionization of the air produced by the charged particles in their transit to the diamond. These small effects may alternatively, or in cooperation with the use of a vacuum, be largely eliminated by mounting the particle source as close as practicable to the diamond, this, therefore, requiring that the diamond 3, source 8, and diaphragm 12 all be very closely interspaced.

Fig. 4 illustrates a preferred embodiment of a pulse in an alternating voltage is employed across the bom-" barded solid insulator. This is an important ingredient of the invention, as pointed out hereinbefore, providing a greatly increased yield over prior disclosed devices of the bombardment conductivity type. It should be understood that no particular alternating voltage wave form is specified, since it may take the form of a sinusoidal wave, square wave, or some other form of which there is a wide choice.

Alternatively, as described in the earlier parts of the specification, the arrangement described in the preceding paragraph may be replaced by a very thin insulator crystal used in conjunction with a high field strength across the same.

In Fig. 4, the vacuum tube closure member 17 is comparable to the closure member of the conventional vacuum tube. A vacuum tube is here necessary, as it was not in the earlier numbered figures, which illustrate apparatus utilizing alpha and beta particles, because here the charged particles are preferably electrons emanating from the usual cathode source as in conventional vacuum tubes. Reference numeral 18 indicates generally an electron gun of a type which is customary in vacuum tubes which generate and utilize a conformed cathode ray or beam.

The gun in fact may conform to a well-known Radio Corporation of America technique, as illustrated in some of their tubes, or in Patent No. 2,458,652 of R. W. Sears, issued January 11, 1949, in which see either Fig. 1 or Fig. 8. The cathode 21 may be indirectly heated, as disclosed, or be of the filamentary type. The electron emanations are urged outwardly and concentrated by anode 22, beyond which the beam passes to the diamond crystal 20 as has been disclosed. Associated with the anode 22, or supplementing it, are the additional focusing and accelerating electrodes and deflection plates which cooperate with each other and with the other gun structure for directing the electron beam. These are described in detail with reference to the bombarding gun disclosed in Figs. 4 and 5 of my application Serial No. 789,667.

All of the structure which affects the electron beam, as shown in Fig. 4 of my application supra, has been found useful by the applicant in the pulse generator circuit of this disclosure, although, as should be obvious, not all of the elements there shown are necessary.

The gun 18 conforms and directs the beam so as to impinge on the solid insulator 20 mounted in the enclosing tube 17 at the opposite end from the said gun. The conditions affecting this solid insulator are similar to those specific to the earlier disclosed species of the invention, the impingement of the electrons being reflected in amplified form as the bombardment induced current inherent in the operation. In a particular experiment by applicant, a diamond was used which was coated with two narrowly separated electrodes constituted by evaporated gold on the diamond face presented to the incident electron beam which were interconnected so as effectively to constitute a single electrode together with one similar electrode on the opposite face of the diamond. Alternatively, other arrangements of dielectric and electrodes may be used within the teachings of the earlier numbered figures, and, in addition, there is a wide choice of solid insulator material.

To the inner, or bombarded, electrode of the insulator 20 is connected the output lead 29, which is connected in series with a high resistance element to ground and across which may be connected a. utilization circuit of the type shown, including, for example, an amplifier 31 and a cathode-ray oscilloscope 32.

To the outer electrode on the face of the insulator 20 is connected the high potential terminal of the alternating-current source 30, the other terminal of which is connected to ground. As has been described in the earlier parts of the specification, the alternating voltage source 30 may suit a variety of conditions as to the particular wave shape of the voltage involved.

Under the conditions of one experiment, that is, with the primary beam current of one microampere, pulse length. of one microsecond, recurrence frequency 4000 cycles per second, an alternating voltage of frequency as low as 20 cycles per second applied across the crystal was suflicient to prevent the formation of large opposing polarization fields which would substantially reduce the internal yield. Thus, the instantaneous current due to conduction by electrons obtained at a given peak alternating voltage applied field was approximately ten times that obtained with the same direct voltage field across the crystal. The maximum value of the voltage which can be applied across the crystal is, as a practical matter, and in the case of a direct voltage source, set by the amplitude of. the initial large fluctuation current through or across the surface of the crystal upon connection of the source to the electrodes. The corresponding maximum peak alternating voltage wasfound to be several times this limiting maximum direct voltage, and this contributed to thefavorable results achieved. For example, using 14- kilovolt bombarding electrons at a peak alternating voltage of about 1170 volts across the crystal, a positive internal yield of the equivalent of 580 conduction electrons passing completely through the crystal for each bombarding electron has been obtained where a positive yield is defined as that in which the conduction electrons travel in the same direction through the crystal as do the bombarding electrons.

The principal structural differences between the amplifier embodiments described with reference to Figs. 4 and 5 in my application Serial No. 789,667 supra and the pulse generator herein described is that the control grid 23 is not connected to a signal source butsimply to a source of negative biasing potential. Moreover, an apertured shield 35, which is of lead or similar material impervious to the bombarding radiation, is mounted within the enclosure 17 closely adjacent the inner surface of the insulator 20. It is maintained at a slightly negative potential with respect to the insulator 20 by connection to a negative biasing source. Anadditional pair of deflecting plates 33 is mounted in a position to control the sweep of the beam across the shield 35 and insulator 20 under control of variations in potential produced by the sweep circuit 34, which may assume any of the forms well known in the art. Accordingly, the organization functions as a prime source of pulses ratherthan as a means for amplifying or conditioning an impressed wave. The electron beam is swept by potentials impressed from sweep facility 34 on deflecting plates 33. The path described by the beam intercepts the defining aperture in the shield 35 which is impervious to the bombarding radiation, thus permitting a short burst of electrons to strike the diamond crystal during each phase of movement of the deflected beam. Accordingly, the bombardment induced conductivity in the crystal 20 and hence in the output lead 29 and associated utilization circuit is of a pulsed character which is determined by the relation between the dimensions of the aperture in shield 35 and the over-all sweep of the beam. The corresponding bombardment induced current in the diamond appearing on the output lead 29 may be amplified and indicated by the oscilloscope 32 or the like.

In an experimental demonstration, a beam of a few microamperes was so deflected. Pulse durations ranging from 4 microsecond up to several milliseconds were obtained, although a much wider range of durations could be obtained. The use of electron bombardment induced'conductivity enables one to use a small primary beam current giving a well focused beam and at the same time permits one to obtain a relatively large pulse in the output circuit. In order to make possible the efficient use of the. oscilloscope in this set-up, the sweep of the beam must be synchronized with or used simultaneously as the sweep for the oscilloscope; that is, a common sweep source may be used, as illustrated. If further, the alternating voltage impressed on the crystal electrodes is synchronized with the sweep frequency through a synchronizing path such as provided by the connecting lead 41, with the additional requirement that provision be made for bombardment of the crystal at some time during both positive and negative half cycles, or multiple thereof, of the alternating voltage, the observed pulses are of equal amplitude. Otherwise, a series of pulses of different amplitudes will be observed, each corresponding to a certain phase of the alternating voltage impressed on the crystal.

A modification of the improved form of bombardment conduction pulse generator described in the preceding paragraphs with reference to Fig. 4 is shown in Fig. 5 of the drawings, which has as its principal feature of difference from the former the replacement of the inner electrode of the crystal insulator target 20 by a low impedance conduction path provided by secondary emission emanating from the target and flowing to a collector electrode. The said secondary emission is induced in the target by means of a low voltage high current beam of electrons focused broadly thereon by a second electron gun included in an extended portion of the envelope enclosing the gun which produces the bombarding beam, the target, and the other elements of the system.

Referring in detail to Fig. 5, the electron gun producing the beam, which is directed to induce secondary radiation in the target 20 and which will be known hereinafter as the holding beam as differentiated from the bombarding beam, comprises a cathode 50, a control grid 36, and a cylindrical focusing electrode 38, all of which are enclosed in the extended portion 39 which extends obliquely outward from the evacuated glass envelope 17 at such an angle as to enable irradiation of the inner surface of the target 20 without presenting interference to the bombarding beam. Such an electron gun may assume any one of a number of forms well known in the art, such as, for example, one of the arrangements described in detail in the aforementioned Patent 2,458,652 to R. W. Sears.

The beam from the cathode 50 is so focused by the positively biased focusing electrode 38 as to broadly cover the inner surface of the insulator element 20. The electrons emanating from the cathode 50, which is usually biased a few thousand volts negative with respect to the insulator 20, provides electrons of suflicient energy to induce secondary emission in the target having a coefficient of the order of unity. Optimum operation is obtained at a point which is known as the second cross-over point, referring to a graphical representation of secondary radiation. The meaning of this phrase may be explained as follows. As the potential applied to the primary electrons of the holding beam is granually increased from zero, the number of secondary electrons emitted by the target also increases, reaching a maximum at which the coeflicient of secondary emission is somewhat in excess of unity, and then gradually decreases, passing a second time through a point at which the coefiicient is unity. The second cross-over point is preferred to the first, since operation of the latter is unstable.

The secondary emission emanating from the target is collected by the electrode 40, which may, for example, take the form of a truncated hollow cone, the inner surface of which is disposed close to the inner face of the insulator 20 to receive the secondary electrons emitted therefrom but which has a sufiiciently wide opening to be substantially out of the impinging path of the bombarding beam. The current density of the holding beam should be sufficient to provide an impedance between the target 20 and the collector 40 which is low compared with the effective impedance set up through the crystal as the result of bombardment. For best operation, the ratio of the current density in the holding 111 beam; to that; of. the bombarding beam should be of tbe orderof 1000 *to'one; Forexample, if the bombardirlg, beamhas-.a-current: density of the order of microamperes, the current density of the holding beam should be of the order. of. milliamperes. In order to maintain. the flow of secondary electrons in the desired direction, the, collector. electrode 401 is biased positively with respect to the target. For example, the cathode 50 maybe maintained at a potential of the order of a kilovolt negative with respect to the target 20, whereas the collectorelectrode is maintained at a potential of the order of a kilovolt positive relative to the said target. Connection is made from the collector 40 through the lead 29'to the output or load resistance, as in the previously described embodiment.

It is apparent that. the bombarding electron gun, the target; the-perforated shield, the sweep circuit, and other elements of the system are similar in structure and functionto like'numbered'elements shown in Fig. 4 described with reference thereto.

The system operates in amanner largely similar to the system described with reference to Fig. 4. When a current. of electrons or holes is. generated in the insulator 20 by. the bombarding beam fromthe collector 21, the potential of the inner surface of the element 20 varies in accordance with variations in the current density of thebeam-as it passes back and forth over the apertured shield 35; audit also varies in accordance with the alternating voltageimpressed from the source 30. In the presently described, embodiment, due to the stimulation producedby theqlow voltage high current beam from the cathode 50, secondary-electrons are emitted from the surface, of insulator 20 and. flow to the positively biased collector 40 until a stableenergy state is established. This secondary electron current,. which is proportional to potential variation across the insulator 20 and'hence is an amplified replica of the pulsed spurts of energy passing through the shield 35 modulated by alternating currentfrom the source 30, passes through the lead 29 into the output circuit.

Such an arrangement hasseveral advantages over the arrangement previously described, namely, that the eifect onthe insulator due to thev bombarding beamis more pronounced with theremoval of the inner electrode which operated to absorbpart of thebombarding radiation and, further, that it is technically simpler in some instances to irradiate the insulator uniformly with electrons than to evaporate or plate a uniform electrode film in contact with the surface.

What is claimed is:

l. Apulse generator comprising in combination a solid electrical insulator, means comprising a source of a high energy beam of electrons for bombarding said insulator with electrically charged particles of sufficient energy to induce electrical conductivity in said insulator, means for reducing the cumulative space charge in said insulator comprising means to apply an electrical biasing field across at least a portion of said insulator causing current carriers to flow in opposite directions across said insulator, means interposed in the path of said beam for determining the quantity of said charged particlesincident on said insulator depending on the angle of incidence of said beam thereon, deflecting means for guiding said beam to sweep intermittently across said insulator and said interposed means, and means responsive to the current pulses generated in said insulator by said swept beam.

2. A pulse generator comprising in combination an electrical insulator, means for applying analternating voltage biasing fieldacross at least a portion of said insulator, means comprising a source of a beam of electrically charged particles for bombarding said insulator, said particles having suflicient energy to remove a large 12 number; of! valence: electrons from their bonds in said insulator, means: interposed in:the path of said beam for determining the quantity. of said charged particlesincident on said insulator depending on the angle of'incidence of: said. beam. thereon, deflecting means for guid ing said beam to sweep-across saidinsulator and said interposed means intermittently, and means responsive to the current. pulses; generated in said insulator by said swept beam.

3. Azpulse generator comprising incombination a solid electrical insulator, means for applying an electric biasing field across at least a portion of said insulator, a source of' a beam of charged particles for bombarding said: insulator with. sufficient energy to render said insulator conducting, means-interposed in the path of said bearmfor determining the quantity of said charged particlesincident on said insulator depending on the angleof incidence of said beam thereon, deflecting means for guiding said: beam to intermittently sweep across said insulator andsaiddnterposedmeans, and means responsive to the. current pulses generated in said insulator bysaid'sweptbeam, wherein the spacing between said field applying meansis so small'as to approach the depth of penetration ofsaid particles, causing the field thereacross, to beextremely large.

4. A pulse generator comprising in combination a solid insulator adapted whenimpressed by a beam of highenergy, electrically charged particles in an electrical field to develop induced electrical conductivity, means'for produeing and directing against said insulator a beam of charged particles, field producing electrodes mounted on said insulator, a source of alternating biasing voltage in circuitwith said electrodes for producing a corresponding fieldtherein, means including an apertured shieldcomprising-a materialrimpervious to said beam interposed in the path of said beam in a plane transverse to the path of said beam, means for biasing said shield substantially: negatively with respect to said insulator, defleeting means including a sweep circuit for periodically sweeping saidbeam across said insulator and said apertured shield, said sweep circuit connected for synchronization by said source of alternating biasing voltage, anda utilization circuit-energetically coupled to said insulator to realize the resultant current pulses induced therein.

5.- A pulse generator comprising in combination a solid insulator adaptedwhen impressed by a beam of high energy; electrically charged particles in an electrical field to develop induced. electrical conductivity, means for producing-and directing against said insulator a beam of chargediparticles, field-producing electrodes mounted'on said insulator, a source of alternating biasing voltage in circuit'with-said electrodes for'producing a corresponding field'therein, an apertured shield impervious to the particles of said b'eam interposed between said source and said-insulator, deflecting means including a sweep circuit responsive to said alternating current biasing source for periodically sweeping" said beam across said insulator and said shield 'in-synchronism with said alternating biasing voltage, and autilization circuit energetically coupled to said insulator to realize the resultant current pulses in ducedtherein.

References Cited in the file of this patent UNITED"STATES PATENTS 2,195,489 Iams Apr. 2, 1940 2,213,547 Iarns Sept. 3, 1940 2,223,001v Farnsworth Nov. 26, 1940 2,307,438,, Whitaker Jan. 5, 19.43. 2,512,655 Kohler June 27, 1950 2,531,600 Barney- Nov. 28, 1950 2,543,039; McKay Feb. 27, 1951 2,547,386" Gray- Apr. 3, 1951 2,548,789 Hergenrother Apr." 10, 1951 

